Telemetry: Use mimimum tracing for timeline benchmarks.

base/message_loop/message_loop.cc

Index: base/message_loop/message_loop.cc
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index 8f6c9d8cb6f0d0901d1a6d7fdb1d04b871adc6a9..6e33b2df34dc7ce28bcb5e133d3ad4bf31d4e36c 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -412,6 +412,12 @@ bool MessageLoop::ProcessNextDelayedNonNestableTask() {
 }
 
 void MessageLoop::RunTask(const PendingTask& pending_task) {
+  // For absolutely minimal tracing, "poll_cpu" captures _everything_
+  // on a typical message loop thread, so long as the thread runs semi-regular
+  // tasks (for sufficient measurement granularity).
+  TRACE_EVENT_END0(TRACE_DISABLED_BY_DEFAULT("poll_cpu"), "Poll");
+  TRACE_EVENT_BEGIN0(TRACE_DISABLED_BY_DEFAULT("poll_cpu"), "Poll");
+
   tracked_objects::TrackedTime start_time =
       tracked_objects::ThreadData::NowForStartOfRun(pending_task.birth_tally);